Optical device for homogenizing a laser beam

ABSTRACT

An optical device for homogenizing a laser beam is described. Such a device is commonly used in the surface treatment of an object. The device consists of front lenses that are bonded together to form an assembly of lenses that breaks the laser light into multiple beams. This results in the homogenization of the gaussian profile of the beam intensity as viewed perpendicular to the beam&#39;s propagation. Advantageously the matrix of lenses is formed using groves or bevels in a manner that minimizes losses resulting from light impinging on glue disposed upon the entire lateral surface of the front lenses. In the invention gaps between front lenses that cause interference with the light path are eliminated. This gap free bonding is accomplished by filling the groves or bevels with glue, inserting a rod or wire into the grove to assemble the front lenses in a support frame, bonding a grill to assembled front lenses, or by using a combination of gluing with a support frame.

FIELD OF THE INVENTION

The present invention concerns a device for homogenizing a laser beam.

It finds a general application in any technical field using a laser, andnotably in laser surface treatment, such as the annealing of amorphoussilicon, cleaning, polishing and surface preparation, notably.

BACKGROUND OF THE INVENTION

In general terms, the energy distribution across the cross section of alaser beam, at least in the lowest type of emission, has a gaussianappearance, which is in any case variable across the cross section ofthe beam.

However, in order to obtain, for example, an optimum annealing ofamorphous silicon, it is necessary to apply a laser beam having ahomogeneous and substantially uniform energy distribution.

This homogeneity is particularly required when the sample to be treatedby laser is of large size (as described in the French Patent Applicationentitled "Device and method for laser surface treatment" filed in thename of the Applicant on Aug. 11, 1995, under the number 95 09778, andis incorporated herein by reference. It is also required when theincident laser beam is a combination of several laser beams coming froma line of laser units arranged in parallel and/or in series (asdescribed in the French Patent Application entitled "Method and devicefor controlling a laser source with several laser units for optimizinglaser surface treatment", also filed in the name of the Applicant onAug. 11, 1995, under the number 95 09780, and is incorporated herein byreference.

Means for homogenizing a laser beam are already known.

For example, the document EP-A-0 266 120 or the document U.S. Pat. No.5,253,110 describes a homogenizer which comprises a matrix of contiguousfront lenses, disposed in m rows and n columns, perpendicularly to thedirection of propagation of the laser beam to be treated, and able todivide the said laser beam into m×n laser beams each having asubstantially geometrical uniform cross section, and a substantiallyhomogeneous energy distribution.

In the document EP-A-0 266 120, at least one convergent collection lens,disposed perpendicularly to the direction of propagation of the lightbeam, downstream of the front lenses in the direction of travel of thelaser beam, focuses, in a given area of a chosen plane, the beams comingfrom the front lenses.

In the documents referred to above, the matrix of lenses comes from asingle piece machined to the format of the said matrix. It gives aplurality of laser beams each having a uniform and substantiallyidentical intensity.

The drawback of such a matrix of lenses, formed of a Single piece, isthat it makes it necessary to change the entire matrix when only one ofthe lenses is defective. In addition, the format of the matrix and thenumber of lenses are fixed. Such a matrix can, therefore, be suited onlyto a single size of laser beam to be homogenized.

Lenses assembled by bonding are known (CN-A-1 052 956), but thisassembly does not deal with all light beams, owing to the glue disposedon the whole lateral surface of the faces of the lenses placed edge toedge. The adhesive is a cause of loss in the light transmission.

Furthermore, it can be destroyed by the power of the incident laser beamwhich irradiates it, lose its bonding properties or cause damage to thelenses.

Lenses are also known which are assembled individually by clamping inhousings (DE-A-3 634 847), but this assembly also interferes with thepath of the light beams, owing to the gaps separating the lenses.

SUMMARY OF THE INVENTION

The present invention affords a solution to these problems. It relatesto an optical device for homogenizing a laser beam.

According to a general definition of the invention, it comprises meansof assembly by bonding and/or clamping able to connect the front lensesto each other individually edge to edge, without gaps, the said assemblymeans comprising adhesive and/or clamping elements as well as groovesand/or bevels formed in the lenses to serve as a housing for the saidadhesive and/or for the said clamping elements so that the said assemblymeans do not interfere with the path of the light beam passing throughthe said front lenses thus connected.

Such a device makes it possible to replace a defective front lenswithout changing the entire set of front lenses. It also improves themaintenance of the said lenses as well as their adaptation to laserbeams of different sizes. In particular, it enables lenses to beassembled individually without loss of light and without interferingwith the travel of the light beams by virtue of the grooves and/orbevels which eliminate the gaps, serving as a housing for the glueand/or clamping elements.

The device according to the invention thus finds an advantageousapplication when the beam to be homogenized is a combination of severallaser beams emanating from a line of laser modules placed in seriesand/or in parallel.

In practice, each front lens has an overall parallelepipedal shape withan entry face, an exit face and four lateral faces. In this case, atleast one lateral face on a front lens designed to be connected to alateral face of an adjacent front lens comprises a transverse grooveand/or a bevel respectively formed in the vicinity of and/or on the exitface of the said front lens.

According to a first embodiment of the invention, the assembly means areby bonding, the grooves and/or bevels being designed to serve as areservoir for adhesive so as to avoid interfering with the path of thelight beams which pass through the lenses thus bonded.

According to a second embodiment of the invention, the assembly meansare by clamping, the grooves and/or bevels being designed for theinsertion of a wire or rod fixed to a frame surrounding the plurality offront lenses.

According to another aspect of the invention, the assembly meanscomprise a grille having a plurality of bars with a width which is smallcompared with that of the front lenses and to which the exit facesand/or bevels of the front lenses are applied by bonding and/orclamping.

In practice, the front lenses are of the convex/plane, biconvex orconvex/concave type, and are convergent or divergent.

Other advantages and characteristics of the invention will be describedin the following detailed description and the drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the essential means of ahomogenizer;

FIG. 2 is a variant of the homogenizer described with reference to FIG.1;

FIG. 3 is a homogenizer with two collection lenses;

FIG. 4 illustrates the connection by bonding of the front lenses of theconvex/plane type according to the invention;

FIG. 5 illustrates the connection by bonding of the front lenses of theconvex/concave type according to the invention;

FIG. 6 illustrates the connection by clamping and/or bonding of thefront lenses of the convex/plane type according to the invention; and

FIG. 7 illustrates the connection by clamping and/or bonding of thefront lenses of the convex/concave type according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, the reference FLA designates the light beam to behomogenized. This laser beam has an energy distribution which isnonuniform in cross section. It emanates for example from a laser orfrom a line of laser modules placed in parallel or in series.

For example, this laser beam is intended for annealing amorphous siliconby laser. One application consists of illuminating a large panel ofamorphous silicon for the manufacture of flat liquid crystal screens.This laser beam is applied to the target plane CIB carrying the sampleto be treated.

The optical device for homogenizing a laser beam comprises here m×ncontiguous front lenses LF, disposed in m rows and n columnsperpendicularly (transversely) to the direction of propagation of thelaser beam to be treated. m and n are integer numbers, for example m andn are equal to 7. The front lenses are arranged in a straight or obliquerectangular matrix. For example, they are evenly distributed within arectangle or parallelogram.

These lenses split the laser beam FLA into m×n laser beams each having asubstantially uniform cross section and a substantially homogeneousenergy distribution.

A convergent collection lens LC is provided, disposed perpendicularly tothe direction of propagation of the light beam, and downstream of thefront lenses LF in the direction of travel of the laser beam. Thiscollection lens is able to focus, in the target plane CIB, the beamscoming from the front lenses.

Advantageously, m×n diaphragms DI are associated respectively with them×n front lenses. Each diaphragm is disposed substantially in the objectfocal plane of the associated lens and receives the light beam comingfrom the associated front lens in order to filter it spatially.

The reference "a" designates the width of a front lens LF. The referencef designates the focal distance (close to the draw tube) between a frontlens and the associated diaphragm DI. The reference F designates thedistance (which can be different from the focal distance) between adiaphragm DI and the collection lens LC. The reference A designates thewidth of the homogenized laser beam obtained by the homogenizer HO atthe plane CIB. The dimension A is equal to a×F/f.

Each diaphragm DI comprises an opening of chosen shape and dimensions,for example circular. A support or mask MA holds the plurality ofdiaphragms.

The front lenses are for example of the convex/plane, biconvex orconvex/concave type, and convergent or divergent.

It is here arranged so that the cross section of the light beam comingfrom each front lens is rectangular or hexagonal, downstream of thediaphragms.

When the collection lens LC is of small diameter, the optical device isless expensive and has fewer optical aberrations than with a largecollection lens.

In FIG. 2, the collection lens LC is disposed so that the focal plane ofthe lenses LF is not merged with that of the collection lens LC. Thisvariant confers a reduction in bulk of the homogenizing device.

The path of the light rays shows here that the homogenizing device isnot necessarily a focal.

With reference to FIG. 3, the collection lens LC is replaced by twoconvergent lenses LC1 and LC2, disposed perpendicularly to the directionof propagation of the light beam, downstream of the front lenses LF andassociated diaphragms DI.

These collection lenses LC1 and LC2 are able to move in translationalong the optical axis.

The distance d between the two lenses LC1 and LC2 is varied in order tovary the resulting focal distance and obtain, at the target plane CIB, asize of homogenized beam which is suited to the chosen application, forexample the size of the panels to be treated in the case of theannealing of amorphous silicon.

It should be noted that the other optical elements, namely the frontlenses LF, the diaphragms DI and the target plane CIB, can also bemovable in translation along the optical axis.

The choice of the relative distances between the different opticalelements makes it possible not only to adapt the size of the homogenizedlaser beam to the chosen application (the possibility of choosing themagnification by varying the distance d), but also to reduce the bulk ofthe homogenizing device.

It should also be noted that the choice of the distance d also makes itpossible to adjust the energy per unit surface delivered by the laserbeam, as described in the patent application filed by the Applicant for"Method and device for controlling a laser source with several laserunits in order to optimize laser surface treatment" as mentioned above.

FIG. 4 depicts the path of several beams through several front lenses LFon the same row m. In order to facilitate an understanding of theinvention, only three front lenses are shown diagrammatically, alongsideeach other. These front lenses are for example of convex/plane type.They are of parallelepipedal shape overall, with an entry face FE, anexit face FS and four lateral faces FL1 to FL4. The convex part (theentry face) receives the laser beam to be treated FLA. One lateral faceof a front lens is defined for example by the points A2, A'3, B'3 and B2for the lateral face FL3 of the central lens LF2.

The laser beam to be treated, coming from the point A1, is applied tothe central lens LF2 at the point A2. This beam passes through the frontlens LF2 as far as the point A3, and is then routed towards the pointA4. It should be noted that the point A3 is slightly offset with respectto the point A'3.

The convex part of a front lens comprises here two sharp edges such asBV.

According to the invention, the front lens LF2 comprises a shallowgroove EN4 formed transversely in the lateral face FL1.

Likewise, the front lens LF2 comprises a shallow groove EN3 formedtransversely in the lateral face FL2.

The grooves are preferably formed close to the exit face of the frontlens.

The same applies to the other front lenses.

The two adjacent front lenses LF2 and LF3 are applied against eachother. The cooperation of the respective grooves EN3 and EN2 enables thesaid lenses to be bonded by applying adhesive in grooves EN3 and EN2,without interfering with the optical properties of the light beamspassing through the said front lenses LF2 and LF3.

The path of the beam B1 through the central front lens LF2 passesthrough the points B2, B3 and B4.

It should be noted advantageously that the light path is 5 not disturbedby the grooves EN and bevels CH.

The front lenses are rectangular or hexagonal in shape overall, withsides with a length of around 1 cm.

For example, the beam to be homogenized FLA is rectangular with sides ofaround 5 to 8 cm.

With reference to FIG. 5, the connection by bonding of the front lensesaccording to the invention can be applied also to divergent front lensesof the convex/concave type.

It should be noted that the grooves described with reference to FIGS. 4and 5 can also serve for the insertion of a wire or rod (not shown)fixed to a frame (not shown) surrounding the matrix of front lenses.These wires or rods thus inserted in the grooves make it possible tojuxtapose and connect together the front lenses. These wires or rods canalso be inserted and held in the grooves by bonding and/or clamping.

With reference to FIGS. 6 and 7, the assembly means comprise a grillehaving a plurality of bars, depicted here at B×1 to B×4, of small widthcompared with that of the front lenses, and to which the exit faces ofthe front lenses are applied by bonding and/or clamping.

The grooves EN described with reference to FIGS. 4 and 5 canadvantageously be omitted in the assembly according to FIGS. 6 and 7.

In FIG. 6, the front lenses are convergent and of the convex/plane type.

As a variant, according to FIG. 7, the front lenses are divergent and ofthe convex/concave type.

It is a question above of homogenizing the laser beam obtained at theexit. Although important applications entail the obtaining of a beamwhose energy density per unit surface is uniform in cross section, it isclear that the word "homogenize" does not necessarily imply equality ofthe energy density over the entire cross section, but on the contraryextends to obtaining any desired distribution of this energy density.

We claim:
 1. An optical device for homogenizing laser light, the device comprising:a plurality of contiguous front lenses with each front lens having a plurality of grooves and bevels formed in the front lenses to facilitate their assembly into a single unit, with the front lenses disposed in m rows and n columns in a plane perpendicular to the direction of propagation of the laser light and dividing the laser light being homogenized into m×n laser beams each having a desired distribution of laser light intensity; and a lens assembly structure for supporting the front lenses and holding them together edge to edge and without gaps, wherein the lens assembly structure does not interfere with the path of the laser light passing through the front lenses.
 2. An optical device as claimed in claim 1, wherein each front lens has an overall parallelepiped shape with an entry face an exit face and four lateral faces.
 3. An optical device as claimed in claim 1, wherein each front lens has an overall hexagon shape with an entry face an exit face and six lateral faces.
 4. An optical device as claimed in claim 1, wherein the lens assembly structure comprises glue disposed in the grooves and bevels.
 5. An optical device as claimed in claim 1, wherein the lens assembly structure comprises multiple supports disposed in the grooves of the front lenses and the supports are attached to a frame surrounding the front lenses such that the front lenses are held together within the frame.
 6. An optical device as claimed in claims 5, wherein the supports are wires.
 7. An optical device as claimed in claim 5, wherein the supports are rods.
 8. An optical device as claimed in claim 1, wherein the lens assembly structure comprises supports disposed in the bevels of the front lenses and wherein the supports are attached to a frame surrounding the front lenses such that the front lenses are held together as a unit.
 9. An optical device as claimed in claim 8, wherein the supports are wires.
 10. An optical device as claimed in claim 8, wherein the supports are rods.
 11. An optical device as claimed in claim 1, wherein the lens assembly structure comprises a grille having a plurality of bars which have a width which is less than the width of the two bevels being covered, the grill being bonded to the bevels at the exit faces of the front lenses.
 12. An optical device as claimed in claim 1, wherein the lens assembly structure comprises a grille having a plurality of bars which have a width that is less than the width of the two bevels being covered, the grille securing the front lenses in place by clamping the grille to the exit faces.
 13. An optical device as in claim 1, wherein the front lenses are arranged and wherein a cross section of laser light emanating from the front lenses in hexagonal.
 14. An optical device as in claim 1, wherein the front lenses contain only bevels.
 15. An optical device as in claim 1, wherein the front lenses are convergent.
 16. An optical device as in claim 1, wherein the front lenses are divergent.
 17. An optical device as in claim 1, wherein the cross section of homogenized laser light coming from the front lenses is rectangular. 